Project Summary RNA-based enzymes play key roles in a wide range of cellular processes, from RNA processing to splicing to translation. While RNA is their catalytic moiety, cellular RNA-based enzymes are ribonucleoproteins (RNPs). The structural and functional roles of proteins and RNA-protein interactions in RNA-based enzymes and RNPs in general are poorly understood. This proposal focuses on S. cerevisiae Ribonuclease (RNase) P and the closely related RNase MRP as a biologically significant and effective model to understand the roles of proteins and RNA-protein interactions in the structure, function, and evolution of ribonucleoproteins. RNases P/MRP are a family of essential RNA-based enzymes that have their origin in the bacterial RNase P ribozyme. In addition to its catalytic RNA moiety, RNase P has a single protein in bacteria, but nine essential proteins in yeast; the protein content increases from ~10% in bacteria to more than 75% in eukaryotes. The reasons for the increased reliance on proteins in the eukaryotic RNase P are not understood, and the functions of the proteins themselves are not known. RNase MRP is an essential eukaryotic RNP. Yeast RNase MRP has a 340-nucleotide-long catalytic RNA which resembles the RNA component of RNase P and 10 essential protein components, eight of which are shared with RNase P. The structural organization of RNase MRP and the roles of its components are not clear. The large and complex eukaryotic RNase P and, especially, RNase MRP are understudied; this proposal aims to fill the gap in our understanding of these fundamentally important catalytic RNPs, and expand our understanding of RNA- protein interactions in general. The specific aims of the proposal are: (1) Characterize respective contributions of proteins and RNA to specificities of RNase P and RNase MRP RNPs; (2) Determine roles of proteins and RNA-protein interactions in the structural organizations of RNases P/MRP and catalysis. We will characterize the RNA-protein interactions in RNases P/MRP using biochemical studies and structural analysis, and clarify the roles of the proteins in RNases P/MRP.